To keep it short and simple: no. Even when the tour has already started, conditions can and will change. Imagine a heavy rain and you would have to stop for an hour or so. That has nothing to do with traffic, yet the navigation time will change.
I would recommend the following: plan stops accurately (e.g. 10 minutes for a photo stop, 5 minutes for refuelling, 30 minutes for lunch break, etc. and enter them in the planner - those figures are just for example). In the end, add another hour for "unforseen" delays and in my humble opinion, you will be fine.
